Field of the Invention
The present invention relates to an image processing technique for reproducing on a recording medium reflection characteristics of an object irradiated with incident light.
Description of Related Art
Spatially Varying Bidirectional Reflectance Distribution Function (SVBRDF) is one technique of describing reflection characteristics of a material with respect to incident light. The SVBRDF describes multi-angular reflection characteristics for each position of a material in accordance with a variation of the angle of incident light/outgoing light. The texture of a material corresponding to a light source/point or view can be reproduced by reproducing the SVBRDF. Yauxiang Lan, Yue Dong, Fabio Pellacini, Xin Tong, in “Bi-Scale Appearance Fabrication”, ACM. Transactions on Graphics(TOG)-SIGGRAPH 2013 Conference Proceedings TOG Homepage archive, July 2013, Volume 32, Issue 4 (hereinbelow, referred to as Xin Tong) discusses a technique of reproducing the SVBRDF on a recording medium. In the technique discussed by Kin Tong, the SVBRDF of a target material is input, and structures approximating the SVBRDF are formed. More specifically, predetermined structures to be formed on the recording medium are prepared, and a combination of the structures is optimized to approximate the reflection characteristics of the target material that have been input.
A characteristic texture of a target material may undesirably be lost in a printed object obtained with the technique discussed by Xin Tong. For example, a material such as velvet features a large difference in color appearance when viewpoint is changed along an elevation angle direction. Thus, the SVBRDF of velvet involves a characteristic reflectance change in the elevation angle direction, but involves no large characteristic reflectance change in an azimuth angle direction. Therefore, reproduction of a velvety texture requires reproduction of the SVBRDF focusing more on the reflectance change in the elevation angle direction than in the azimuth angle direction. However, the optimization in the technique discussed by Xin Tong simply focuses on reduction in the difference from the SVBRDF regardless of the elevation angle direction or the azimuth angle direction despite the risk of approximation error, without taking the characteristics of the actual material (e.g., velvet, as described above) into consideration. Therefore, such whole optimization may fail to sufficiently reproduce the texture of certain materials, such as a velvet cloth. More specifically, the characteristic reflectance change in the elevation angle direction may fail to be sufficiently reproduced due to an influence of reflectance in the azimuth angle direction which is not a characteristic feature of the texture of the material. The same applies to a texture of other materials such as metal (metallic) and satin.